TRS-80 Fireworks

Tony Duell ard.p850ug1 at
Wed Aug 28 11:23:41 CDT 2019

On Wed, Aug 28, 2019 at 5:06 PM Ethan O'Toole via cctalk
<cctalk at> wrote:
> We were at this HOPE conference in NYC. We were helping with it in those
> days, so we had some equipment at the ops area. A whole bunch of commotion
> went down in the OPS area, including someone grabbing a fire extingisher
> and jumping.
> Watching this from a-far, we chuckled and I said, "Good thing this doesn't
> involve us."
> A few seconds later someone tells me that my TRS-80 model 4 started to let
> out a bunch of magic smoke.
> I mention it to one of the VCF'ers that was there. I'm pretty sure it was
> Glitch. He is like, "Oh that's the rice paper capacitor at postiion C103
> or something. A noise filter."
> "Do you work on these often?" I reply. I figured this was the ninja of
> TRS-80s or something. "No, pretty much never" he says.
> Now I'm twice as confused. But whatever. We open the thing up and it is
> the exact capacitor he mentioned. We just rmeoved it. And I'm still
> confused as how he knew the exact capacitor that went off the top of his
> head.

FWIW my Model 3 and Model 4 have done just that. As did my Acorn
Cambridge when I was setting it up for a demonstration about a month
ago. It's a common failure in all sorts of equipment, from all manufacurers.
Don't think that Radio Shack are using inferior components here, I've had
HP and Tektronix units do the same thing.

Although I don't replace components for fun, I normally replace all the
capacitors in the mains filter (class X across the mains, class Y from
either side of the mains to earth ground). The class Y ones don't normally
fail in this way, but....

As for knowing which capacitor it was even without familiarity with the
unit. The smell and smoke are quite distinctive. And the macbine normally
keeps on running, indicating a mains filter problem most of the time.

Now for the secret... The mains filter is the first stage of the power supply.
So it has the lowest numbered capacitors. There's normally a filter choke
unit with a capacitor across the mains on each side. And a pair of capacitors
on one side of that from each mains line to ground. So they are likely to be
C1...C4 or C101...C104 or something like that.

So guess 'C103 or thereabouts' and you won't be far off :-)


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